Recently, it is desired to improve image quality in an image that is formed on a recording medium, particularly plain paper, by an ink jet recording method, specifically, the gradation of the image, and color uniformity. For example, the gradation can be improved by decreasing area per one dot formed by the ink on the recording medium. On the contrary, if the area per dot is large, a plurality of dots are easily connected, so that the area of a surface of the recording medium that is covered by the ink may not be precisely controlled even if the amount of the applied ink per unit area is increased or decreased. Also, since the plurality of dots is easily connected, the blur of the ink on the recording medium may easily occur.
For such problems, it is proposed to suppress the blur of the ink on the recording medium by increasing the static surface tension (so-called “surface tension”) of the ink. For example, it is proposed to suppress blur by setting the surface tension of the ink at 20° C. to 45 mN/m or more (see Japanese Patent Application Laid-Open No. S63-213581). Also, it is proposed to suppress blur by setting the surface tension of the ink to 40 mN/m or more (see Japanese Patent Application Laid-Open No. 2004-83621).
Also, as technology for controlling the blur of the ink on the recording medium, the permeation of the ink into the recording medium, and the like, proposition focusing on the dynamic surface tension of the ink is made. For example, it is proposed to suppress blur by setting surface tension at bubble period T (seconds/bubble)≦0.2 to 40 mN/m or more, and to improve ejection reliability by setting surface tension at bubble period T (seconds/bubble)>1 to less than 50 mN/m (see Japanese Patent Application Laid-Open No. H09-296139).
Also, it is proposed to improve ejection reliability and define the dynamic surface tension property at specific lifetime by setting the dry viscosity of the ink to 100 mPa·s or less (see Japanese Patent Application Laid-Open No. 2003-231838). Specifically, it is described that blur is suppressed by setting dynamic surface tension at a lifetime of 10 milliseconds to 45 mN/m or more and that the quick-drying property is improved by setting dynamic surface tension at a lifetime of 1000 ms to 35 mN/m or less.